Paper mills have for many years made extensive use, for the screening of paper making stock, of screening apparatus embodying a cylindrical perforate screen member defining screening and accepts chambers on the opposite sides thereof in a closed housing and provided with a rotor member which operates in one of the chambers to keep the screen perforations open and free from solid materials tending to cling to the screen surface. In operation, the stock or furnish is delivered to the screening chamber adjacent one end of the screen member, and the material rejected by the screen member is collected and discharged from the opposite end of the screen member.
The assignee of this invention has manufactured and sold many such screens, originally in accordance with Staege U.S. Pat. No. 2,347,716, and subsequently in accordance with Martindale U S. Pat. No. 2,835,173, the latter construction being characterized by a rotor comprising bars or vanes of air-foil section in closely spaced but non-contacting relation with the surface of the screen member. Similar screens have been marketed for some years, in competition with those of the assignee of this invention, in accordance with other patents, such as Cannon et al U.S. Pat. Nos. 2,975,899, Lamort 3,617,008 and Holz 3,581,983.
The art has also experimented widely with detailed variations in screens of the above type, including variations in the size, spacing and configuration of the perforations in the screen member and also in the vane shape and in other forms of rotor. For example, such screens have been offered in recent years wherein the rotor is a drum-like member provided with multiple bumps or other offset portions over its surface. Typical such constructions are shown in Clarke-Pounder U.S. Pat. Nos. 3,363,759 and Bolton et al 3,726,401.
In all of the vertically oriented commercial screens of the type outlined above, the primary direction of through flow is downwardly, with the stock entering the screen chamber from above, or in some cases centrally of the screening chamber when the direction of screening is from the outside to the inside of the screen member, so that any high specific gravity reject material entrained with the stock to be screened will travel by gravity to a reject discharge chamber in the lower part of the screen, from which it is subsequently discharged. Necessarily, therefore, there is substantial opportunity for such reject material to damage the perforate screen member as it travels through the screening chamber, especially with screens of the type wherein the screening chamber is on the inside of the perforate screen member, and wherein centrifugal force therefore will cause high specific gravity materials to travel along the screening surface.
Another type of reject material, which is becoming increasingly prevalent in waste paper stocks, is material of lower specific gravity than the paper fibers, such as pieces of plastic, especially scraps of plastic foam. The circulatory movement imparted to the stock in the screening chamber by whatever rotor is used will develop centrifugal force which will tend to cause such light materials to migrate toward the center of the screening and reject chambers. However, in order to discharge these concentrated light reject materials, it is necessary for them to overcome the gravitational forces which tend to cause them to rise within the apparatus and therefore away from the reject discharge outlet.
Weber U.S. Pat. No. 4,166,068 discloses a different construction of screening apparatus of the general type outlined above wherein the supply flow of stock to be screened enters the apparatus by way of an inlet chamber located entirely below the screening chamber, and wherein low specific gravity reject materials, including materials of substantially the same specific gravity as accepted fiber, are collected in a reject chamber above the screening chamber, and from which they are discharged by a port located generally centrally of the top wall of the apparatus in order to ensure effective removal of light reject materials of the types discussed above.
In the apparatus of the Weber patent, the high specific gravity reject materials entering through the stock inlet are retained in the inlet chamber by constructing the inlet and screening chambers so as to provide an annular space in the inlet chamber which is of greater outer diameter than the flow passage through which the stock enters the screening chamber, and producing sufficient centrifugal force in the inlet chamber to cause these high specific gravity materials, such as tramp metal and the like, to be collected in this annular space and thereby to prevent them from coming into contact with the screen member Such trapped high specific gravity materials are discharged from time to time directly from this annular space by the reject outlet means, so that only materials of approximately the same specific gravity as paper fiber or a lower specific gravity are allowed to reach the screening chamber.